Pathways to Paraptosis After ER Photodamage in OVCAR ‐5 Cells

Targeting paraptosis in cancer: opportunities and challenges

Cell death can be classified into two primary categories: accidental cell death and regulated cell death (RCD). Within RCD, there are distinct apoptotic and non-apoptotic cell death pathways. Among the various forms of non-apoptotic RCD, paraptosis stands out as a unique mechanism characterized by distinct morphological changes within cells. These alterations encompass cytoplasmic vacuolization, organelle swelling, notably in the endoplasmic reticulum and mitochondria, and the absence of typical apoptotic features, such as cell shrinkage and DNA fragmentation. Biochemically, paraptosis distinguishes itself by its independence from caspases, which are conventionally associated with apoptotic death. This intriguing cell death pathway can be initiated by various cellular stressors, including oxidative stress, protein misfolding, and specific chemical compounds. Dysregulated paraptosis plays a pivotal role in several critical cancer-related processes, such as autophagic degradation, drug resistance, and angiogenesis. This review provides a comprehensive overview of recent advancements in our understanding of the mechanisms and regulation of paraptosis. Additionally, it delves into the potential of paraptosis-related compounds for targeted cancer treatment, with the aim of enhancing treatment efficacy while minimizing harm to healthy cells.

Metal-based nanoparticles in cancer therapy: Exploring photodynamic therapy and its interplay with regulated cell death pathways

Photodynamic therapy (PDT) represents a non-invasive treatment strategy currently utilized in the clinical management of selected cancers and infections. This technique is predicated on the administration of a photosensitizer (PS) and subsequent irradiation with light of specific wavelengths, thereby generating reactive oxygen species (ROS) within targeted cells. The cellular effects of PDT are dependent on both the localization of the PS and the severity of ROS challenge, potentially leading to the stimulation of various cell death modalities. For many years, the concept of regulated cell death (RCD) triggered by photodynamic reactions predominantly encompassed apoptosis, necrosis, and autophagy. However, in recent decades, further explorations have unveiled additional cell death modalities, such as necroptosis, ferroptosis, cuproptosis, pyroptosis, parthanatos, and immunogenic cell death (ICD), which helps to achieve tumor cell elimination. Recently, nanoparticles (NPs) have demonstrated substantial advantages over traditional PSs and become important components of PDT, due to their improved physicochemical properties, such as enhanced solubility and superior specificity for targeted cells. This review aims to summarize recent advancements in the applications of different metal-based NPs as PSs or delivery systems for optimized PDT in cancer treatment. Furthermore, it mechanistically highlights the contribution of RCD pathways during PDT with metal NPs and how these forms of cell death can improve specific PDT regimens in cancer therapy.

An ultrasound-activatable platinum prodrug for sono-sensitized chemotherapy

Despite the great success achieved by photoactivated chemotherapy, eradicating deep tumors using external sources with high tissue penetration depth remains a challenge. Here, we present cyaninplatin, a paradigm of Pt(IV) anticancer prodrug that can be activated by ultrasound in a precise and spatiotemporally controllable manner. Upon sono-activation, mitochondria-accumulated cyaninplatin exhibits strengthened mitochondrial DNA damage and cell killing efficiency, and the prodrug overcomes drug resistance as a consequence of combined effects from released Pt(II) chemotherapeutics, the depletion of intracellular reductants, and the burst of reactive oxygen species, which gives rise to a therapeutic approach, namely sono-sensitized chemotherapy (SSCT). Guided by high-resolution ultrasound, optical, and photoacoustic imaging modalities, cyaninplatin realizes the overall theranostics of tumors in vivo with superior efficacy and biosafety. This work highlights the practical utility of ultrasound to precisely activate Pt(IV) anticancer prodrugs for the eradication of deep tumor lesions and broadens the biomedical uses of Pt coordination complexes.

V. J, Pal S, Nair BG, Kar R, Mishra N. Paraptosis: a unique cell death mode for targeting cancer

Programmed cell death (PCD) is the universal process that maintains cellular homeostasis and regulates all living systems' development, health and disease. Out of all, apoptosis is one of the major PCDs that was found to play a crucial role in many disease conditions, including cancer. The cancer cells acquire the ability to escape apoptotic cell death, thereby increasing their resistance towards current therapies. This issue has led to the need to search for alternate forms of programmed cell death mechanisms. Paraptosis is an alternative cell death pathway characterized by vacuolation and damage to the endoplasmic reticulum and mitochondria. Many natural compounds and metallic complexes have been reported to induce paraptosis in cancer cell lines. Since the morphological and biochemical features of paraptosis are much different from apoptosis and other alternate PCDs, it is crucial to understand the different modulators governing it. In this review, we have highlighted the factors that trigger paraptosis and the role of specific modulators in mediating this alternative cell death pathway. Recent findings include the role of paraptosis in inducing anti-tumour T-cell immunity and other immunogenic responses against cancer. A significant role played by paraptosis in cancer has also scaled its importance in knowing its mechanism. The study of paraptosis in xenograft mice, zebrafish model, 3D cultures, and novel paraptosis-based prognostic model for low-grade glioma patients have led to the broad aspect and its potential involvement in the field of cancer therapy. The co-occurrence of different modes of cell death with photodynamic therapy and other combinatorial treatments in the tumour microenvironment are also summarized here. Finally, the growth, challenges, and future perspectives of paraptosis research in cancer are discussed in this review. Understanding this unique PCD pathway would help to develop potential therapy and combat chemo-resistance in various cancer.

Combination of Two Photosensitisers in Anticancer, Antimicrobial and Upconversion Photodynamic Therapy

Photodynamic therapy (PDT) is a special form of phototherapy in which oxygen is needed, in addition to light and a drug called a photosensitiser (PS), to create cytotoxic species that can destroy cancer cells and various pathogens. PDT is often used in combination with other antitumor and antimicrobial therapies to sensitise cells to other agents, minimise the risk of resistance and improve overall outcomes. Furthermore, the aim of combining two photosensitising agents in PDT is to overcome the shortcomings of the monotherapeutic approach and the limitations of individual agents, as well as to achieve synergistic or additive effects, which allows the administration of PSs in lower concentrations, consequently reducing dark toxicity and preventing skin photosensitivity. The most common strategies in anticancer PDT use two PSs to combine the targeting of different organelles and cell-death mechanisms and, in addition to cancer cells, simultaneously target tumour vasculature and induce immune responses. The use of PDT with upconversion nanoparticles is a promising approach to the treatment of deep tissues and the goal of using two PSs is to improve drug loading and singlet oxygen production. In antimicrobial PDT, two PSs are often combined to generate various reactive oxygen species through both Type I and Type II processes.

Which cell death modality wins the contest for photodynamic therapy of cancer?

Photodynamic therapy (PDT) was discovered more than 100 years ago. Since then, many protocols and agents for PDT have been proposed for the treatment of several types of cancer. Traditionally, cell death induced by PDT was categorized into three types: apoptosis, cell death associated with autophagy, and necrosis. However, with the discovery of several other regulated cell death modalities in recent years, it has become clear that this is a rather simple understanding of the mechanisms of action of PDT. New observations revealed that cancer cells exposed to PDT can pass through various non-conventional cell death pathways, such as paraptosis, parthanatos, mitotic catastrophe, pyroptosis, necroptosis, and ferroptosis. Nowadays, immunogenic cell death (ICD) has become one of the most promising ways to eradicate tumor cells by activation of the T-cell adaptive immune response and induction of long-term immunological memory. ICD can be triggered by many anti-cancer treatment methods, including PDT. In this review, we critically discuss recent findings on the non-conventional cell death mechanisms triggered by PDT. Next, we emphasize the role and contribution of ICD in these PDT-induced non-conventional cell death modalities. Finally, we discuss the obstacles and propose several areas of research that will help to overcome these challenges and lead to the development of highly effective anti-cancer therapy based on PDT.

Detection of Paraptosis After Photodynamic Therapy

Photodynamic therapy (PDT) is a procedure for the selective photosensitization of neoplastic tissues. Subsequent irradiation with visible light can lead to cell death along with vascular shutdown and other responses that lead to selective eradication of malignant cells. Among the cellular responses to PDT are necrosis, apoptosis, and autophagy. These pathways have generally been associated with cell death, although autophagy can also be cytoprotective. A fourth effect that has hitherto been somewhat neglected is termed "paraptosis," a lethal response that can be identified by detecting an extensive collection of cytoplasmic vacuoles. Unlike autophagy, these vacuoles are bound by single membranes. Paraptosis has been characterized as a response to misfolded endoplasmic reticulum proteins that must be "cleared" if the affected cell is to survive. At present, there is no simple biochemical test for paraptosis. This chapter describes the procedure for detection of paraptosis using phase-contrast microscopy, along with some confirmatory approaches.

A Promising Anticancer Agent Dimethoxycurcumin: Aspects of Pharmacokinetics, Efficacy, Mechanism, and Nanoformulation for Drug Delivery

Curcumin is a well-known anticancer natural product with various significant bioactivities that has been well documented, but its widespread use is mainly hindered by insufficient ADME properties such as poor solubility and low metabolic stability. Dimethoxycurcumin (DiMC) is a kind of lipophilic compound derived from curcumin that maintains its anticancer potency and has greatly improved systematic bioavailability. Therefore, DiMC is regarded as a promising plant-derived anticancer agent that deserves to be well developed. Herein, we concentrate on the published work by those from original research groups concerned with the pharmacokinetics, efficacy, and mechanism of DiMC involved in the treatment of various tumors, as well as the nanoformulations for effective drug delivery.

Characteristics of an Impaired PDT Response

A concurrent human papilloma virus (HPV) infection potentiates the efficacy of ionizing radiation for treatment of head and neck cancer by promoting apoptosis. Studies in cell culture indicated an opposite effect for photodynamic therapy (PDT) when this leads to mitochondrial and ER photodamage. The explanation for this difference in PDT efficacy remains to be established. While apoptosis was impaired in HPV(-) cells, such cells can be killed via photodamage directed at the ER: this leads to a nonapoptotic death pathway termed paraptosis. No differences in photosensitizer uptake or reactive oxygen species (ROS) production were observed in HPV(+) vs. HPV(-) tumors. We now provide evidence that death pathways initiated by ER/mitochondrial photodamage leading to either paraptosis or apoptosis are impaired in an HPV(+) head and neck cell line. These results illustrate the complex determinants of PDT efficacy, a topic that has yet to be fully explored.

Factors Affecting Photodynamic Therapy and Anti-Tumor Immune Response

Photodynamic Therapy (PDT) is a cancer therapy involving the systemic injection of a Photosensitizer (PS) that localizes to some extent in a tumor. After an appropriate time (ranging from minutes to days), the tumor is irradiated with red or near-infrared light either as a surface spot or by interstitial optical fibers. The PS is excited by the light to form a long-lived triplet state that can react with ambient oxygen to produce Reactive Oxygen Species (ROS) such as singlet oxygen and/or hydroxyl radicals, that kill tumor cells, destroy tumor blood vessels, and lead to tumor regression and necrosis. It has long been realized that in some cases, PDT can also stimulate the host immune system, leading to a systemic anti-tumor immune response that can also destroy distant metastases and guard against tumor recurrence. The present paper aims to cover some of the factors that can affect the likelihood and efficiency of this immune response. The structure of the PS, drug-light interval, rate of light delivery, mode of cancer cell death, expression of tumor-associated antigens, and combinations of PDT with various adjuvants all can play a role in stimulating the host immune system. Considering the recent revolution in tumor immunotherapy triggered by the success of checkpoint inhibitors, it appears that the time is ripe for PDT to be investigated in combination with other approaches in clinical scenarios.

Paraptosis after ER Photodamage Initiated by m-tetra(hydroxyphenyl) Chlorin

Two cell lines, A549 (human-derived nonsmall-cell lung cancer) and 1c1c7 (mouse hepatoma), were photosensitized with m-THPC and irradiated under LD₉₀ conditions. After 4 h, a pattern of cytoplasmic vacuoles had formed consistent with the initiation of paraptosis. After irradiation, there was no detectable loss of the mitochondrial membrane potential indicating no significant photodamage to mitochondria. We did, however, observe localization of m-THPC in the endoplasmic reticulum (ER), as indicated by fluorescence microscopy. Subsequent ER perturbation is known to result in initiation of paraptosis, another pathway to cell death. While an apoptotic response to m-THPC has been reported, the ability to target ER and induce paraptosis could explain the efficacy of this agent which could therefore eradicate cell types with an impaired apoptotic response.

Death Pathways Associated with Photodynamic Therapy

This report describes studies involving ER vs. lysosomal targeting and is designed to assess the initiation of different death pathways as a function of subcellular targeting and PDT dose. Photodamage directed at mitochondria or lysosomes initiates apoptosis, a death pathway generally considered to be irreversible. Photodamage that involves the ER can lead to another death pathway termed paraptosis. This does not involve caspase activation, can eradicate cell types with impaired apoptosis; at high levels of irradiation, apoptosis and necrosis were observed. Autophagy has a cytoprotective function unless lysosomes are targeted; loss of lysosomal integrity can interfere with the autophagic recycling processes.

Photodynamic therapy, priming and optical imaging: Potential co-conspirators in treatment design and optimization - a Thomas Dougherty Award for Excellence in PDT paper

Photodynamic therapy is a photochemistry-based approach, approved for the treatment of several malignant and non-malignant pathologies. It relies on the use of a non-toxic, light activatable chemical, photosensitizer, which preferentially accumulates in tissues/cells and, upon irradiation with the appropriate wavelength of light, confers cytotoxicity by generation of reactive molecular species. The preferential accumulation however is not universal and, depending on the anatomical site, the ratio of tumor to normal tissue may be reversed in favor of normal tissue. Under such circumstances, control of the volume of light illumination provides a second handle of selectivity. Singlet oxygen is the putative favorite reactive molecular species although other entities such as nitric oxide have been credibly implicated. Typically, most photosensitizers in current clinical use have a finite quantum yield of fluorescence which is exploited for surgery guidance and can also be incorporated for monitoring and treatment design. In addition, the photodynamic process alters the cellular, stromal, and/or vascular microenvironment transiently in a process termed photodynamic priming, making it more receptive to subsequent additional therapies including chemo- and immunotherapy. Thus, photodynamic priming may be considered as an enabling technology for the more commonly used frontline treatments. Recently, there has been an increase in the exploitation of the theranostic potential of photodynamic therapy in different preclinical and clinical settings with the use of new photosensitizer formulations and combinatorial therapeutic options. The emergence of nanomedicine has further added to the repertoire of photodynamic therapy's potential and the convergence and co-evolution of these two exciting tools is expected to push the barriers of smart therapies, where such optical approaches might have a special niche. This review provides a perspective on current status of photodynamic therapy in anti-cancer and anti-microbial therapies and it suggests how evolving technologies combined with photochemically-initiated molecular processes may be exploited to become co-conspirators in optimization of treatment outcomes. We also project, at least for the short term, the direction that this modality may be taking in the near future.

Photodynamic Nanophotosensitizers: Promising Materials for Tumor Theranostics

Photodynamic theranostics/therapy (PDT) is a potential strategy for selectively imaging malignant sites and treating cancer via a non-invasive therapeutic method. Photosensitizers, the crucial components of PDT, enable colocalization of photons and light, and photon/light therapy in the therapeutic window of 400-900 nm exhibits photocytotoxicity to tumor cells. Due to their high biostability and photocytotoxicity, nanophotosensitizers (NPSs) are of much interest for malignant tumor theranostics at present. NPS-activated photons transfer energy through the absorption of a photon and convert molecular oxygen to the singlet reactive oxygen species, which leads to apoptosis and necrosis. Moreover, NPSs modified by polymers, including PLGA, PEG-PLA, PDLLA, PVCL-g-PLA, and P(VCL-co-VIM)-g-PLA, exhibit excellent biocompatibility, and a tumor-targeting molecule linked on the nanoparticle surface can precisely deliver NPSs into the tumor region. The development of NPSs will accelerate the progress in tumor theranostics through the photon/light pathway.

Exploring Modes of Photokilling by Hypericin

Effects of photodynamic therapy (PDT) using the anthraquinone hypericin were explored with OVCAR-5 cells in vitro. Irradiation resulted in ER > lysosomal photodamage. Paraptosis was identified as a primary death pathway resulting from ER perturbation. This is characterized by an extensive pattern of cytoplasmic vacuole formation. As the PDT dose increased, apoptotic death was also detected. The cytoprotective effect of autophagy, observed when certain other subcellular sites are PDT targets, appears to be absent. These results, together with prior evidence that paraptosis can be lethal to cells with an impaired apoptotic pathway, suggest a role for agents with this targeting profile in photodynamic therapy. A limitation to be overcome for hypericin is a suboptimal absorbance profile.

Paraptosis and Photodynamic Therapy: A Progress Report

Photodamage to the endoplasmic reticulum (ER) can initiate a death pathway termed paraptosis. The "canonical" model of paraptosis, initiated by certain drugs and other stimuli, requires a brief interval of protein synthesis, involves the action of MAP kinases and can be followed by biochemical markers. The latter include changes in expression of AIP-1/Alix and IGF-1R proteins and translocation of HMGB-1 from nucleus to plasma membrane. There is also a report indicating that an enhanced level of autophagy can impair death by paraptosis. The pathway to paraptosis follows the canonical pathway when ER photodamage is minor (<LD₅₀ ). When the extent of ER photodamage approaches LD₉₀ levels, there are deviations from the "canonical" pathway: interfering with protein synthesis does not prevent paraptosis nor does a brief chilling of cells after irradiation, MAP kinases are not involved, and stimulation of autophagy was not cytoprotective. We had previously speculated that ER protein cross-linking might potentiate paraptosis (Photochem. Photobiol. 95, 2019, 1239) but this appears to be incorrect. At higher PDT doses, substantial cross-linking of a typical ER protein (BiP, binding immunoglobin protein, an HSP chaperone) was detected and paraptosis was impaired. This may relate to decreased mobility of cross-linked proteins. Other pathways to cell death were then observed.

Zinc(II) phthalocyanines as photosensitizers for antitumor photodynamic therapy

Photodynamic therapy (PDT) is a highly specific and clinically approved method for cancer treatment in which a nontoxic drug known as photosensitizer (PS) is administered to a patient. After selective tumor irradiation, an almost complete eradication of the tumor can be reached as a consequence of reactive oxygen species (ROS) generation, which not only damage tumor cells, but also lead to tumor-associated vasculature occlusion and the induction of an immune response. Despite exhaustive investigation and encouraging results, zinc(II) phthalocyanines (ZnPcs) have not been approved as PSs for clinical use yet. This review presents an overview on the physicochemical properties of ZnPcs and biological results obtained both in vitro and in more complex models, such as 3D cell cultures, chicken chorioallantoic membranes and tumor-bearing mice. Cell death pathways induced after PDT treatment with ZnPcs are discussed in each case. Finally, combined therapeutic strategies including ZnPcs and the currently available clinical trials are mentioned.